1. Field of the Invention
The present invention relates to a rotational speed detector which is incorporated in an antiskid brake system (hereinafter referred as simply "ABS") for an automobile to detect a rotational speed of a wheel when braked in order to prevent locking of the wheel during braking (i.e., the phenomenon that the wheel is completely prevented from rotating although the automobile is still being driven), or which is designed to detect a rotational speed of each of the shafts that are incorporated in an automatic transmission in order to obtain a timing at which the shafts are to be locked up (i.e., the shafts are mechanically coupled together).
2. Description of the Prior Art
It is common practice to detect a rotational speed of a shaft in a variety of machinery. In ABS, for example, a rotational speed of a wheel when braked has heretofore been detected by a rotational speed detector, e.g., those disclosed in Japanese Patent Post-Exam. Publication No. 52-46331 (corresponding U.S. Pat. No. 4,069,435) and Japanese Patent Public Disclosure (KOKAI) No. 62-249069.
The rotational speed detector that is disclosed in Japanese Patent Post-Exam. Publication No. 52-46331 is arranged as shown in FIG. 1.
Referring to FIG. 1, reference numeral 1 denotes a hub, and a flange 2 for securing a wheel is rigidly secured to the outer peripheral surface of the hub 1. The outer peripheral surface of the hub 1, which is a rotating member, is formed with outward tracks 3. An outer ring 4 is supported by a suspension system, and rolling elements 5 are disposed between the outward tracks 3 formed in the outer peripheral surface of the hub 1 and inward tracks 6 that are formed in the inner peripheral surface of the outer ring 4, thereby enabling the hub 1 to rotate inside the outer ring 4.
A gear-shaped sensor rotor 7 is rigidly secured to one end portion of the hub 1, and a rotational speed sensor 8 is provided on a part of a cover 9 which faces the sensor rotor 7, the cover 9 being secured to one end portion of the outer ring 4, so that it is possible to detect a rotational speed of the hub 1 having the sensor rotor 7 secured thereto.
In the rotational speed detector that is disclosed in Japanese Patent Public Disclosure (KOKAI) No. 62-249069, as shown in FIG. 2, a sensor rotor 7 is secured to the outer peripheral surface of the intermediate portion of a hub 1, and one end portion of a rotational speed sensor 8 which extends through an outer ring 4 faces the outer peripheral edge of the sensor rotor 7.
In these known rotational speed detectors, as the sensor rotor 7 rotates together with the hub 1, the output voltage from the rotational speed sensor 8, which is provided in opposing relation to the sensor rotor 7, changes.
The frequency of the output voltage from the rotational speed sensor 8 that changes with the rotation of the sensor rotor 7 secured to the hub 1 is proportional to the number of revolutions of the hub 1. Therefore, if the output signal from the rotational speed sensor 8 is inputted to a controller (not shown), it is possible to determine the number of revolutions (rotational speed) of the wheel that is secured to the hub 1.
There has heretofore been another type of rotational speed detector wherein, as shown in FIG. 3, a sensor rotor 12 which comprises a short cylinder-shaped body 10 with a multiplicity of equally spaced through-holes 11 is firmly fitted around the outer peripheral surface of a rotating member 13, and a rotational speed sensor 15 is provided on a housing-like member 14, for example, an outer ring, which surrounds the periphery of the rotating member 13 in such a manner that the sensor 15 faces the sensor rotor 12.
However, the conventional rotational speed detectors such as those described above involve the following problems:
In the prior art, the sensor rotor 7 is secured to a rotating member, for example, the hub 1, and the rotational speed sensor 8 is secured to a stationary member, for example, the outer ring 4, in such a manner that the sensor rotor 7 and the rotational speed sensor 8 face each other at a predetermined distance. Therefore, under certain circumstances, the output voltage from the rotational speed sensor 8 is not constant.
The voltage of a signal that is delivered from the rotational speed sensor 8 varies greatly with the distance between the rotational speed sensor 8 and the sensor rotor 7, and this distance changes subtly due to errors in manufacturing the hub 1, the outer ring 4 and the sensor rotor 7. Even when the manufacturing errors of these members 1, 4 and 7 are so small that they may be neglected, if the hub 1 and the outer ring 4 are elastically deformed by the stress that is applied to these members 1 and 4 during the running of the automobile, the distance between the rotational speed sensor 8 and the sensor rotor 7 changes temporarily.
If the change in the distance between the rotational speed sensor 8 and the sensor rotor 7 is small and the voltage of the signal delivered from the rotational speed sensor 8 is adequate for the detection, no particular problem arises. However, if this distance increases by an excessive amount, the absolute value of the output signal voltage becomes inadequate to detect a number of revolutions of the wheel by the controller, even if it is a temporary phenomenon.